1. Field of the Invention
The present invention relates to a method and apparatus for ink jet printing, and more specifically, to a method and apparatus for ink jet printing which carries out printing by attaching a reacting liquid and coloring inks to a print medium.
2. Description of the Related Art
An ink jet printing system, an electrophotographic system, a thermal head system, and the like have been utilized as a printing system for image printing apparatuses represented by printers. Of these printing systems, the ink jet system can be used to increase the quality of images outputted by the image printing apparatus and to facilitate coloring of images. Further, coloring inks for ink jet printing and print media such as paper are both relatively inexpensive and various print medium types are available. Consequently, image printing apparatuses utilizing such an ink jet printing system are widely utilized at general homes, offices, and the like.
The ink jet printing system causes small droplets of coloring inks for printing to fly and adhere to a print medium. In particular, Japanese Patent Application Publication Nos. 61-059911 (1986), 61-059912 (1986), and 61-059914 (1986) disclose a method of using an electrothermal converter as means for supplying ejection energy. This method thermally changes the state of the coloring inks to eject the inks from ejection ports on the basis of the change in state, thus forming droplets. This method provides a print head with a high-density multi-orifice. Thus, images of high resolution and high quality can be printed at high speed.
However, in general, the coloring inks used for the conventional ink jet printing mainly consist of water and contain a water-soluble high-boiling-point solvent such as glycol in order to prevent drying and clogging. If such coloring inks are used to execute printing on ordinary paper, they permeate through the ordinary paper to hinder a sufficient image density from being achieved. In some cases, the image density becomes ununiform (or non-uniform) expectedly because of the ununiform distributions, on a surface of print paper, of a loading material and a sizing agent contained in the coloring inks. Further, if a color image is to be obtained, coloring inks for multiple colors are sequentially superimposed on one another before being fixed to the print medium. Accordingly, in a boundary portion of images formed by different colors, bleeding may occur in which the colors bleed and are ununiformly mixed together. Then, a satisfactory image cannot be obtained.
As means for correcting the bleeding, a method has been disclosed which attaches a liquid serving to improve an output image from the image printing apparatus (this liquid will hereinafter referred to as a “reacting liquid”), to a print medium before ejection of the coloring inks. Japanese Patent Application Laid-open No. 5-202328 (1993) proposes a method of preventing bleeding utilizing the reaction between polyvalent metal ions and carboxyl groups. Further, Japanese Patent Application Laid-open No. 9-207424 (1997) proposes a method of correcting bleeding using the reaction between a pigment and a resin emulsion and a polyvalent metal salt.
Thus, for the method of ink jet printing in which the reacting liquid is attached to a print medium before the coloring inks, a number of methods for efficiently carrying out printing have been proposed. Japanese Patent Application Laid-open No. 7-195823 (1995) proposes an ink jet printing apparatus in which a reacting liquid ejecting nozzle is placed at a leading end of a print head of a serial printer in a main scanning direction so that the reacting liquid is attached to the surface of a print medium before the coloring inks. However, when this configuration is used for bidirectional printing in order to increase a printing speed, the order in which the reacting liquid and the coloring inks are attached to the print medium is reversed between a forward scan and a backward scan. As a result, ununiformity of colors occurs at each scan of certain forward and backward scanning, thus degrading the quality of the image.
On the other hand, Japanese Patent Application Laid-open No. 2001-138554 proposes an ink jet printing apparatus in which the reacting liquid ejecting nozzles are provided at both ends of the print head in the scanning direction so as to enable high-grade images to be printed at high speed through one-pass bidirectional printing. However, in this apparatus, since the reacting liquid ejecting nozzles are provided at both ends, an additional chip and an additional recovery unit must be provided. This increases costs and complicates the apparatus.
Moreover, Japanese Patent Application Laid-open No. 10-193579 (1998) proposes an inkjet print head characterized in which the reacting liquid ejecting nozzle is placed in front of coloring ink ejecting nozzles in a direction in which the print medium is moved in a paper feeding operation. If this ink jet print head is used to carry out one-pass bidirectional printing, since the reacting liquid ejecting nozzle is disposed in front of the coloring ink ejecting nozzles in the paper feeding direction, the coloring inks impact, during a backward scan, the reacting liquid applied to the print medium during a forward scan. Alternatively, the coloring inks impact, during a forward scan, the reacting liquid applied to the print medium during a backward scan. On this occasion, before dots of the coloring inks ejected during a second scan following a first scan impact dots of the reacting liquid impacting the print medium at a certain point during the first scan, a difference in impacting time occurs which corresponds to the time required by the ink jet print head to move to one end of the print medium and return to the initial point. The difference in impacting time varies, within a band, depending on the position impacted by the reacting liquid. Consequently, even within the same band, the difference in impacting time is large in some parts and small in other parts. Thus, the colors may be ununiform within the band.
If bidirectional printing is carried out using the reacting liquid in order to prevent bleeding caused by the coloring inks as described above, the order in which the reacting liquid and the coloring inks are attached to the print medium is reversed between the forward scan and backward scan of the print head as disclosed in Japanese Patent Application Laid-open No. 7-195823 (1995). As a result, ununiformity of colors occurs at each scan of certain forward and backward scanning, thus degrading the quality of the image. On the other hand, Japanese Patent Application Laid-open No. 2001-138554 solves this problem. However, since this apparatus is provided with the reacting liquid ejecting nozzles at both ends of the print head, an additional chip and an additional recovery unit must be provided. This increases costs and complicates the apparatus.
Japanese Patent Application Laid-open No. 10-193579 (1998) is a configuration which solves the above problem and in which the print head has a reacting liquid nozzle placed upstream in the paper feeding direction and coloring ink ejecting nozzles arranged downstream in the paper feeding direction.
However, according to Japanese Patent Application Laid-open No. 10-193579 (1998), the amount of reacting liquid differs from the amount of the coloring inks reacting within the same band of the print medium depending on the difference between the time at which the reacting liquid impacts the print medium and the time at which the coloring inks impact the print medium. Thus difference in the reacting amount may result in nonuniform colors.